Method for the mass spectrometric analysis of solids

ABSTRACT

There is disclosed a method for the mass spectrometric analysis of solids using a spark ion source. In one embodiment a plurality of spark discharges are provided for each exposure of an ion sensitive layer, resulting in a corresponding plurality of ion pulses wherein a predetermined number of successive ion pulses out of a given number thereof are suppressed. For example, only one in every one hundred pulses may be used for exposure, whereas the other ninety nine pulses are suppressed. In an alternate embodiment, a predetermined major proportion of the ions constituting each pulse are suppressed, the remainder being allowed to pass to an analysis part of the spectrometer.

[111 3,809,896 1451 May 7, 1974 METHOD FOR THE MASS SPECTROMETRIC ANALYSIS OF SOLIDS Inventors: Klaus Dieter Schuy, Am Dobben;

Jochen Franzen, Witten/Ruhr, both of Germany Varian Mat GmbH, Bremen, Germany Filed: May 25, 1971 Appl. No.: 146,693

US. Cl. ..250/286 1111.121 B0ld 59/44 F ield ofSearch..250/4l.9 SA, 41.9 SB, 41.9 SE,

250/419 SR, 41.9 D, 43.5 R

[56] References Cited UNITED STATES PATENTS 3,370,171 2/1968 Ohta 250/41.9 SA 3,600,573 8/1971 Watanabe 250/419 SA FOREIGN PATENTS OR APPLICATIONS 1,189,419 4 1970 Great Britain 250/419 SA Primary Examiner-James W. Lawrence Assistant Examiner-D. C. Nelms There is disclosed a method for the mass spectrometric analysis of solids using a spark ion source. In one embodiment a plurality of spark discharges are provided for each exposure of an ion sensitive layer, resulting in a corresponding plurality of ion pulses wherein a predetermined number of successive ion pulses out of a given number thereof are suppressed. For example, only one in every one hundred pulses may be used for exposure, whereas the other ninety nine pulses are suppressed. In an alternate embodiment, a predetermined major proportion of the ions constituting each pulse are suppressed, the remainder being allowed to pass to an analysis part of the spectrometer.

ABSTRACT 7 Claims, 7 Drawing Figures GEN PULSE $0 24 OUNTER METHOD FOR THE MASS SPECTROMETRIC ANALYSIS OF SOLIDS RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 748,539, filed July 29, 1968 and now abandoned.

FIELD OF THE INVENTION The present invention relates to a method of massspectrometric analysis and an apparatus for use in performing the method.

BACKGROUND OF THE INVENTION It is customary in connection with the analysis of solids, using spark ion sources, to record the mass spectra on ion sensitive layers, e.g., photographic plates. The mass spectra is recorded on the ion sensitive layer in a series of differently strongly stepped exposures, the number of successive spark discharges used for the exposures being increased in a stepwise manner from exposure to exposure. It has been shown particularly in the case of exposures with a small number of discharges that the precision of the analysis is generally very unsatisfactory.

The problem underlying the invention is to provide a method of spark source mass spectrographic analysis, which ensures a high and as regular as possible a precision of the analysis, for all exposure steps.

In the solution of this problem, the invention proceeds from the consideration that the sample quantities used in mass spectroscopic analysis of solids, with spark ion sources, are extremely small, so that for precision analysis, high requirements are placed on the homogeneity of the sample being investigated. These homogeneity requirements are greater the smaller the number of discharges used for the exposure. This explains the fact that with a low number of exposures, large analysis inaccuracies occur in the previously used methods, since the composition of ion currents emitted by only a few discharges is not representative of the ion current composition nor of the sample composition, as when a large number of discharges occurs.

SUMMARY OF THE INVENTION For avoiding this disadvantage, it has been found that the utilization of the sample during the individual exposures, more particularly during small exposures, can be increased. This is achieved according to the invention by operating with very many more discharges than are necessary for the desired exposure, and suppressing the ions not required for the desired exposure. The procedure can be such that out of very many discharges, only one discharge is used for the exposure. For performing such a method, the generator for operating the spark ion source is dimensioned for a discharge rate which is greater than the number of discharges or ion current pulses required for the exposure. A counting unit is provided for signals given by the spark generator, which controls a device for excluding the ion current pulses from the exposure, and is adjusted in such a manner that only the number of ion current pulses necessary for the exposure degree desired in any case is used, the remaining ion current pulses being rendered ineffective for the exposure.

A still better increase in precision can be achieved when using a larger number of discharges for the exposure by using only a fraction of the ions emitted by each individual discharge for the exposure, the main part of the discharge being excluded from the exposure.

The masking out of the exposure, of individual ion current pulses from a plurality of discharges, and the masking of a fraction of the ions emitted by each individual discharge, can be effected with the use of a known device for deflecting the ion beam out of the path to the ion sensitive layer. In the first case, this device is controlled by a pulsecounter, in the second case by a circuit or device which renders accessible for the mass spectrometric analysis the ions emitted during a correspondingly pre-selectable time interval from each individual discharge. The ions emitted during the remainder of the discharge period are suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS In order to make the invention clearly understood, reference will now be made to the accompanying drawings which are given by way of example and in which:

FIG. 5 is a diagram representing operation in accordance with another mode of operation of the invention;

FIG. 6 is a fragmentary diagram usable with the major parts of the diagram of FIG. 1 and showing additional circuitry for practicing the second mode of operation in accordance with this invention; and

FIG. 7 is a waveform associated with the circuitry of FIG. 6.

DETAILED DESCRIPTION OF THE DRAWINGS In FIG. 1, only those parts of the mass spectrometer which are essential to the invention are illustrated, namely the spark ion source 1 and the ion-optical system extending as far as the entry gap 19 to the analysis part 3 of the mass spectrometer.

The spark ion source consists essentially of two electrodes 4, 5 of which one or both consists of the solid substance to be analysed.

In the illustrated embodiment, as a pulse generator for the externally triggered low voltage discharge, a storage chain 6 is used which is connected through resistances 7 and 8 and the diode paths 9 and 10 to the two electrodes 4 and 5 defining the discharge path.

The pulse shaped discharges of this storage chain are triggered by ignition pulses from a pulse generator 11 through a discharge circuit 12 and coupled to the spark path via transformer 13.

The electrodes 4 and 5 of the spark path and the housing 14 surrounding the ionization chamber are at a high positive potential of e.g. +20 kV relative to a diaphragm 16 which is at earth potential, so as to produce an electric ion-acceleration field. Housing 14 has an ion exit gap 15 disposed therein. The ions are directed in the form of an essentially parallel bundled ion beam 18 through slit 17 of diaphragm 16 to the entry gap 19' of the diaphragm 20 lying in front of the analysis part 3 of the mass spectrometer.

A pair of plates 21, 22 are provided between diaphragms l6 and 20 for establishing an electric field for deflecting the ion beam 18. Plate 21 is grounded and the other plate 22 is connected to the output of an electronic counter 23 which may be of conventional design. The input of counter 23 is connected through a con denser 24 to diode path and from there receives a counting pulse on the occurrence of each discharge pulse coupled via transformer 13.

The counter 23 is so arranged that its output normally supplies a deflection voltzgeof e.g. 100 volts to the deflection plate 22 and thus deflects the ion beam 18 so that it deviates from its path through the entry grap l9 and instead impinges against diaphragm 20.

The pulse generator ll operates with a pulse sequence which is as high in frequency as allowed by the ion source. For example, a pulse spacing of one millisecond or a pulse frequency of one hundred pulses per second may be used. Most of the ion current pulses are not used for the exposure, since most of the ion current pulses are deflected against the diaphragm so that they cannot pass through the gap 19. Only on every nth pulse, e.g., every 100th pulse, is the deflection voltage of I00 volts removed from the plate 22 for a short time by counter 23, so that during this time the ions can pass between the plates 21 and 22 without deflection by these plates. The ions then may pass through the gap 19 into the analysis part 3 and thus be used for the exposure.

For a clearer understanding of the mode of operation in accordance with the present invention reference is now made to FIGS. 2-5. The diagram of FIG. 2 corresponds to a prior art mode of operation such as one disclosed in US. Pat. No. 3,370,171. The method there described shows no interruption of the ion beam during each period of exposure. The solid pulsesoccuring during time period T1 represent the actual sequential ion beam pulses used for the purpose of exposure. The interruption in the ion beam takes place only after each exposure has been completed. These interrupted pulses are shown .in dash line in FIG. 2. This mode of operation causes inaccuracies, as explained herein before, in

. the mass spectroscopic analysis.

FIG. 3 shows another method of operation disclosed in US. Pat. No. 3,370,l7 I. In accordance with this method, the ionization itself is interrupted at the end of each exposure by influencing the spark ion source by the degree of exposure which is measured at a restricting electrode. In US Pat. No. 3,3 70,17 1 this electrode is referred to as slit member 8. Referring again to FIG. 2 there is shown only one of the exposure steps with, for example, there pulses needed for the exposure. Exposures with one and two pulses may be preceded, and exposures with four and more pulses may stepwise follow the exposure illustrated in FIG. 2. Depending on the interval between the two following exposures, one, two or more individual pulses may be shielded by the shutter up to the next exposure as indicated by the dash lines in FIG. 2.

Returning again to FIG. 3 however, according to this method, it is necessary to use six pulses instead of three pulses, if only half of the ion current pulsed through the restricting electrode reaches the photographic plate.

This effect however is only combined with the need to use a certain part of the ion current for measuring the degree of exposure. It would be impractical to use such a method as indicated in FIG. 3 for increasing the interval of exposure to a degree as is reached by the present invention.

Two modes of operation in accordance with the present invention are represented in FIGS. 4 and 5. In FIG. 4 only one of a predetermined group of ion pulses is admitted to the input of the mass spectrometer. This pulse is shown in solid whereas the other pulses are shown in dotted. FIG. 4 illustrates still another approach in which only a portion of each pulse is admitted to the input of the mass spectrometer. The portion of the pulse that is admitted is shown in solid and the remainder of the pulse is shown in dotted. FIG. 5 actually shows two different modes of operation also in that the admitted portion of the pulse is at the tail end of the pulse using one approach, and. is intermediate the boundaries of the pulse using another approach.

In FIGS. 4 and 5 it is noted that the periods of exposure namely periods T3 and T4, respectively, are longer than the periods T1 and T2 shown in FIGS. 2 and 3, respectively. This provides for an increased accuracy in the analysis of the mass spectrum.

Referring now to FIG. 6 there is shown circuitry that may replace the counter 23 of FIG. 1 to thereby provide operation of the type depicted in FIG. 5. The signal coupled by way of capacitor 24 is fed to an exposure control device 26 the output of which couples to plate 22 located in the mass spectrometer. It is the purpose of device 26 to control the time at which, and the duration of time thatm ions are permitted to flow through gap 19. This is controlled electrically by forcing plate 22 to a ground potential for a short time duration during the ion pulse.

The exposure control device 26 includes a pair of monostable multivibrators M1 and M2. These multivibrators are connected. in series between capacitor 24 and plate 22. As illustrated in the waveform of FIG. 7 the output of multivibrator M1 may be adjusted to have an output of At while the output of multivibrator M2 may be adjusted to have an output width of A1 By varying the pulse output of multivibrator M1 the energizing time of multivibrator M2 may be varied. By varying the output width of multivibrator M2 the time interval during which plate 22 is ,at ground may be varied. The construction of the multivibrators M1 and M2 would be such that the output of multivibrator M2 would always be at its positive level and would go to essentially ground potential for a short period to thereby' enable ions from the ion beam to flow through gap 19.

pair of electrodes to produce the sequence of spark discharges B. means defining the input of a mass spectrometer,

and

C. means for intermittently excluding and admitting ions to the input of the mass spectrometer during first and second time intervals respectively, the means for intermittently excluding and admitting including l. a diaphragm at the input of the mass spectrometer, the diaphragm having an aperture for admitting ions to the mass spectrometer, 2. deflection means for directing the entire beam away from the aperture in the first time intervals and directing the beam through the aperture during the second time intervals, 3. and a timer responsive to the electrical pulses of 1 the pulse source, the timer being coupled to the deflection means for controlling the admission of ions to the input of the mass spectrometer.

2. The invention according to claim 1, wherein the timer includes,

counting means for repeatedly causing an ion pulse in the beam to enter the input to the mass spectrometer after a predetermined number of preceding ion pulses have been deflected away from the aperture in the diaphragm.

3. The invention according to claim 1 wherein the timer includes,

exposure control means for repeatedlyadmitting ions to the input of the mass spectrometer during a predetermined portion of each ion pulse and excluding ions to the input of the mass spectrometer for the remaining portion of the ion pulse,

and where the remaining portion is greater than the predetermined portion.

4. The invention according to claim 3 wherein the exposure control means includes,

a first monostable multivibrator for controlling the time of initiation of the predetermined portion of the ion pulse,

and a second monostable multivibrator coupled in series with the first monostable multivibrator for controlling the duration of the predetermined portion.

5. A method for mass spectroscopic analysis of a solid substance which method includes the steps of subjecting the solid substance to a sequence of spark discharges in an ionization chamber to cause the substance to ionize,

forming a beam of ions from the ionized material, the beam being a train of ion pulses and being directed toward an aperture in a diaphragm at the entrance of a mass spectrometer,

and deflecting the beam in timed relation to the occurrence of the spark discharges to cause the beam intermittently to pass through the aperture in the diaphragm and at other times to have the entire beam impinge upon the diaphragm whereby only portions of the material ionized during the sequence of spark discharges are admitted to the mass spectrometer.

6. The method of claim 5 wherein the beam is deflected to pass through the aperture during an ion pulse after a predetermined number of preceding ion pulses have been deflected away from the aperture.

7. The method of claim 5 wherein a predetermined portion smaller than the entire time duration of each ion pulse, is deflected to pass through the aperture, the remainder of the ion pulse being deflected away from the aperture. 

1. Apparatus for mass spectroscopic analysis of a solid substance, the apparatus comprising A. means for generating a sequence of spark discharges to ionize the substance and produce a beam of ion pulses, the means including
 1. a spark source having a pair of electrodes
 2. a pulse source for generating a train of electrical pulses
 3. means for applying the electrical pulses to the pair of electrodes to produce the sequence of spark discharges B. means defining the input of a mass spectrometer, and C. means for intermittently excluding and admitting ions to the input of the mass spectrometer during first and second time intervals respectively, the means for intermittently excluding and admitting including
 1. a diaphragm at the input of the mass spectrometer, the diaphragm having an aperture for admitting ions to the mass spectrometer,
 2. deflection means for directing the entire beam away from the aperture in the first time intervals and directing the beam through the aperture during the second time intervals,
 3. and a timer responsive to the electrical pulses of the pulse source, the timer being coupled to the deflection means for controlling the admission of ions to the input of the mass spectrometer.
 2. a pulse source for generating a train of electrical pulses
 2. The invention according to claim 1, wherein the timer includes, counting means for repeatedly causing an ion pulse in the beam to enter the input to the mass spectrometer after a predetermined number of preceding ion pulses have been deflected away from the aperture in the diaphragm.
 2. deflection means for directing the entire beam away from the aperture in the first time intervals and directing the beam through the aperture during the second time intervals,
 3. means for applying the electrical pulses to the pair of electrodes to produce the sequence of spark discharges B. means defining the input of a mass spectrometer, and C. means for intermittently excluding and admitting ions to the input of the mass spectrometer during first and second time intervals respectively, the means for intermittently excluding and admitting including
 3. and a timer responsive to the electrical pulses of the pulse source, the timer being coupled to the deflection means for controlling the admission of ions to the input of the mass spectrometer.
 3. The invention according to claim 1 wherein the timer includes, exposure control means for repeatedly admitting ions to the input of the mass spectrometer during a predetermined portion of each ion pulse and excluding ions to the input of the mass spectrometer for the remaining portion of the ion pulse, and where the remaining portion is greater than the predetermined portion.
 4. The invention according to claim 3 wherein the exposure control means includes, a first monostable multivibrator for controlling the time of initiation of the predetermined portion of the ion pulse, and a second monostable multivibrator coupled in series with the first monostable multivibrator for controlLing the duration of the predetermined portion.
 5. A method for mass spectroscopic analysis of a solid substance which method includes the steps of subjecting the solid substance to a sequence of spark discharges in an ionization chamber to cause the substance to ionize, forming a beam of ions from the ionized material, the beam being a train of ion pulses and being directed toward an aperture in a diaphragm at the entrance of a mass spectrometer, and deflecting the beam in timed relation to the occurrence of the spark discharges to cause the beam intermittently to pass through the aperture in the diaphragm and at other times to have the entire beam impinge upon the diaphragm whereby only portions of the material ionized during the sequence of spark discharges are admitted to the mass spectrometer.
 6. The method of claim 5 wherein the beam is deflected to pass through the aperture during an ion pulse after a predetermined number of preceding ion pulses have been deflected away from the aperture.
 7. The method of claim 5 wherein a predetermined portion smaller than the entire time duration of each ion pulse, is deflected to pass through the aperture, the remainder of the ion pulse being deflected away from the aperture. 